Process for producing a zirconia refractory heating element

ABSTRACT

A process for producing a laminate-shaped zirconia refractory heating element which generates heat by passing a current therethrough or by induction heating including the steps of incorporating a flexible binder in a filler composed of zirconia fibers and stabilized zirconia powder, forming the mixture into a refractory sheet having flexibility at room temperature, laminating the refractory sheet to form a multi-layer and firing the laminate.

This is a division of application Ser. No. 306,441, filed Feb. 3, 1989,now U.S. Pat. No. 5,073,68.

BACKGROUND OF THE INVENTION

The present invention relates to a ceramic heating element and, moreparticularly, to a zirconia fiber-based or zirconia fiber-reinforcedzirconia heating element which generates heat by passing a current or byinduction heating, and a process therefor. The present invention alsorelates to a heating structure composed of such a zirconia heatingelement and a lead member for passing a current and a process therefor.

Heretofore, the heating elements used in oxidizing atmospheres havevaried depending upon the temperatures used. In general, SiC heatingelements have been used at temperatures up to about 1,400° C., and MoSi₂heating elements have been used at temperatures up to 1,700° C. Heatingelements capable of being used at higher temperatures have not beenwidely utilized. While heating elements of La₂ O3-Cr₂ O₃ system havebeen partially used, it is not widespread because La₂ O₃ is expensive,is difficult to prepare a large sintered body and because Cr₂ O₃evaporates with use of the heating elements, to contaminate the innerportion cf a furnace.

On the other hand, there have been developed resistance heating unitscomposed of negative characteristic elements exhibiting electricconductivity in the vicinity of 1,000° C. by heating ceramic materialscontaining zirconia (ZrO₂) or thoria (ThO₂) and other additives such ascalcia (CaO) or yttria (Y₂ O₃). (Japanese Patent Publication No.12330/1963 or the like). Ultra-high temperature furnaces composed ofresistance heating units have been already partially practicallyutilized It is expected that resistance heating units will become widelyutilized to produce high melting point single crystals, to producespecial high temperature materials, or to study physical properties athigh temperatures.

Of these, zirconia heating elements are heating materials obtained byadding a small amount of specific oxides to zirconium oxide (ZrO₂) andsintering the mixture at a high temperature Because the melting point ofzirconia (ZrO₂) is 2,690° C., zirconia heating elements can afford hightemperature of up to 2,400° C.

However, the prior zirconia heating elements often fail due to the factthat current is locally passed. Their heat conductivity is low and theircoefficient of expansion is large. Accordingly, the prior zirconiaheating elements exhibit low thermal shock resistance and tend to bedamaged by shock. Accordingly, zirconia heating elements have not beensubstantially produced.

Further, heretofore, in order to form zirconia heating structures fromthese zirconia heating elements, the following methods have beenutilized: a method wherein pores are formed in said heating elementbody, platinum wires passed through the pores as lead members forpassing a current, and entwined to fix them; another method wherein aplatinum paste is applied to the heating element body and this heatingelement body is joined to lead members for passing a current atdiffusion areas as wide as possible.

However, the former method poses a relaxation problem in use, whereasthe latter method puts large restrictions on the form of the furnaces.Further, these methods can fail due to the fact that a current islocally passed through the contact portion between the lead member forpassing a current, and the heating element body.

There has also been proposed a method of securing a heating element bodyand a lead member for passing a current by means of a castablerefractory. However, when both members are secured by means of the priorcastable refractory, they cannot follow in thermal expansion orshrinkage generated by passing a current, and the contact resistanceassociated with the generation of cracks is increased. Thus, the leadmember for passing the current is overheated and damaged, and thereforethe prior product cannot be used as a heating element in many cases.

An object of the present invention is to provide a zirconia heatingelement which generates heat by passing a current or by inductionheating and a process therefor, wherein the zirconia heating element hasexcellent exothermic characteristics when a current is passed andwherein it has resistance to thermal shock.

Another object of the present invention is to provide a process forproducing a zirconia heating element which has excellent current-passingexothermic characteristics or induction heating characteristics andresistance to thermal shock wherein the variation of thesecharacteristics is little if damage should occur, and wherein thezirconia heating element can be readily produced.

A further object of the present invention is to provide a practicalzirconia heating structure obtained by joining a heating element bodyand a lead member for passing a current using a composition exhibitingconductivity at a high temperature which can strongly join the zirconiaheating element electrically and mechanically and the lead member forpassing a current through the zirconia heating element wherein the leadmember has sufficient conductivity even at room temperature; and aprocess therefor.

SUMMARY OF THE INVENTION

An attempt has been made to improve the characteristics of zirconiaheating elements. We have now found that, when a zirconia heatingelement composed of a zirconia fiber or reinforced with the zirconiafiber is used, the objects of the present invention are effectivelyachieved.

Thus, in a first aspect, the present invention relates to a zirconiarefractory heating element which generates heat by passing a current orby induction heating, said zirconia refractory heating elementcomprising a zirconia fiber.

In a preferred embodiment, a zirconia fiber is used as a reinforcingagent. In this embodiment, 100 parts by weight of a zirconia powder areincorporated in from 5 to 1,000 parts by weight of a zirconia fiber.

An attempt has also been made to improve the characteristics of arefractory heating element. We have now found that, when a zirconiafiber-reinforced flexible refractory sheet is used as a stock andlaminated in the form of a roll or multilayer to produce a zirconiaheating element, the objects of the present invention are achievedparticularly effectively.

Thus, in a second aspect, the present invention relates to a process forproducing a zirconia refractory heating element which comprises thesteps of incorporating a flexible binder in a filler composed of azirconia fiber and zirconia powder to which a zirconia stabilizer hasbeen added; forming the mixture into a refractory sheet havingflexibility at room temperature; winding and laminating said refractorysheet in the form of a roll or laminating said refractory sheet in theform of a multilayer; and firing the laminate.

In a preferred embodiment of the process according to the presentinvention, in laminating the refractory sheet, the attachment side of alead wire for passing a current can be fixed to the refractory sheet andthereafter the said sheet can be laminated.

Further, we have discovered that, when a specimen obtained by forming azirconia curing composition described hereinafter into a rod and firingthe rod is used to examine its electrical resistance at hightemperatures, it has sufficient conductivity of from about 3 to about 50Ω·cm at 1,700° C. We have now found that the specimen can be utilized inorder to join a lead member for passing a current (a wire rod, rod orplate) to a zirconia heating element.

Thus, in a third aspect, the present invention relates to a zirconiaheating structure which generates heat by passing a current, saidstructure comprising a zirconia heating element and a lead member forpassing a current, wherein the junction portions between said zirconiaheating element and said lead member for passing the current and theperipheries thereof are coated with a zirconia curing compositioncomprising a zirconia fiber, a zirconia powder, a water-solublezirconium salt as a binder (its aqueous solution exhibiting acidity),and yttria or a water-insoluble yttrium compound as a curing agent tojoin the zirconia heating element and the lead member; and to a processtherefor.

In a preferred embodiment, the zirconia heating structure can be furtherimpregnated with a zirconia sol and/or a solution of a zirconiumcompound after the zirconia curing composition is cured and fired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the shape of a heating element usedin a service test;

FIG. 2 is a perspective view showing the shape of a heating element ofanother embodiment used in a service test;

FIG. 3 is a perspective view showing the state wherein a heating elementis subjected to induction heating;

FIG. 4 is a sectional view taken along line 4--4 of FIG. 3;

FIGS. 5 and 6 are views showing the steps for producing a heatingelement from a refractory sheet;

FIG. 7 is a perspective view showing the state wherein a heating elementis disposed at a holding fixture;

FIG. 8 (a) and (b) are views illustrating a junction site of a leadmember and a zirconia heating element described in Example 10;

FIG. 9 is a view illustrating a junction site of a lead member and azirconia heating element described in Example 11;

FIG. 10 is a view illustrating a junction site of a lead member and azirconia heating element described in Example 12;

FIG. 11 (a), (b) and (c) are views illustrating a junction site of alead member and a zirconia heating element described in Example 13.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in more detail.

[A] Zirconia Heating Element (1) Zirconia Fiber

Zirconia fibers used in the present invention substantially consist ofzirconium oxide represented by the chemical formula: ZrO₂. A zirconiumcompound such as zirconium carbonate or zirconium hydroxide, or amixture of the zirconium compound and a stabilizer such as Y₂ O₃, MgO orCaO can be optionally incorporated in the zirconia fibers. Examples ofthe zirconia fibers which can be used in the present invention includepure zirconia fibers, lime-stabilized zirconia fibers, magnesia-addedzirconia fibers, yttria-added zirconia fibers, ceria-stabilized zirconiafibers, gadolia-stabilized zirconia fibers and mixtures thereof.Preferred zirconia fibers are zirconia fibers to which yttria has beenadded.

The zirconia fibers can be prepared by various methods. For example,zirconia fibers can be prepared by using an aqueous solution of azirconium compound as a starting material (a spinning solution),fiberizing the aqueous solution to form a fiber precursor and firing thefiber precursor. The zirconia fibers can be suitably selected dependingupon the uses and shapes of zirconia refractories.

The length and diameter of the zirconia fibers for use herein are, forexample, from 0.1 to 50 millimeters and from 0.1 to 20 micrometers,respectively.

(2) Zirconia Powder

The zirconia powder used in a preferred embodiment of the presentinvention substantially consists of zirconium oxide. A zirconiumcompound such as zirconium carbonate or zirconium hydroxide, or amixture of the zirconium compound and a stabilizer such as yttrium,magnesium or calcium can be optionally incorporated in the zirconiapowder. The zirconia powder can be obtained by spray drying to a finepowder. The grain size of the zirconia powder is not particularlyrestricted in the present invention and can be suitably selecteddepending upon the uses and shapes of the zirconia refractory heatingelements. For example, the particle diameter of the zirconia powder canbe from 0.1 to 1,000 micrometers, preferably from 0.5 to 500micrometers.

When the zirconia powder is incorporated in the zirconia fiber, theamount of the zirconia fiber is from 5 to 1,000 parts by weight,preferably from 10 to 100 parts by weight, and more preferably from 30to 70 parts by weight based on 100 parts by weight of the zirconiapowder. If the amount of the zirconia fiber added is less than the lowerlimit, a fiber-added effect will be small.

(3) Binder

Examples of the binders which can be used include synthetic polymerssuch as polyethylene oxide, polyvinyl alcohol and polyacrylic acid;cellulose derivatives such as methyl cellulose, carboxyethyl cellulose,hydroxymethyl cellulose, hydroxyethyl cellulose and cellulose phosphate;animal and plant viscous materials such as starch, its derivatives,pectin, sodium alginate, and agar; as well as a zirconia sol and/or anaqueous solution of a zirconium compound wherein such a sol or aqueoussolution converts into zirconia by firing it.

A preferred binder is the zirconia sol and/or the aqueous solution ofthe zirconium compound wherein the sol or aqueous solution converts intozirconia by firing it. In a preferred embodiment of the presentinvention, the zirconia sol is, for example, a milky colloidal solutionwherein zirconia having a particle diameter of about 70 millimicrons issuspended in water. For example, an aqueous solution of a zirconium saltcan be used as the aqueous solution of the zirconium compound Examplesof the zirconium salts include zirconyl acetate, zirconyl nitrate,zirconyl oxychloride, zirconyl sulfate, and ammonium zirconyl carbonate.Hydrolyzates of zirconium alkoxides such as tetrapropyl zirconate andtetrabutyl zirconate can also be used as the aqueous solution of thezirconium compound. These are converted into zirconia by firing them.The binder can be incorporated in the starting material components.Alternatively, molded products can be immersed in a binder solution.

(3-a) Zirconia Stabilizer

Other binders for use herein are crystal stabilizers for zirconia orprecursors which convert into crystal stabilizer by heating. Thecrystal-stabilizing binders are metal oxides which exhibit zirconiacrystal stabilization and/or metal salts which form metal oxides byheating. Examples of the binders include oxides, carbonates, basiccarbonates, acetates, oxalates, nitrates, chlorides, and sulfates ofmagnesium, yttrium, calcium, gadolinium, cerium, samarium, cadmium,lanthanum, and neodymium. Powders of oxides, chlorides, carbonates,basic carbonates of magnesium, calcium and yttrium are preferred fromthe standpoint of economy.

The grain size of the binder of the metal oxides and metal salts is from0.01 to 1,000 micrometers, preferably from 0.1 to 300 micrometers. Ifthe grain size is less than the lower limit, migration phenomenon tendsto occur on drying. If the grain size exceeds the upper limit, theperformance of the binder will decrease and the strength will belowered.

The amount of the crystal-stabilizing binder added is from 0.5 to 30parts by weight, preferably from 1 to 20 parts by weight (on an oxidebasis) based on 100 parts by weight of the zirconia fiber. If the amountof the crystal-stabilizing binder is less than the lower limit, thestrength of a zirconia fiber molded product will be obtained. If theamount of the crystal-stabilizing binder is more than the upper limit,metals and glasses will not be wetted and the characteristics of azirconia fiber molded product (i.e., excellent conductivity, chemicallystablility and corrosion resistance) will be impaired.

(4) Additive

In addition to the components described above, various additives can beused herein depending upon purposes. Examples of such additives includepore-forming agents, surfactants, dispersants, flocculants and crystalstabilizers.

In order to decrease the weight of the zirconia heating element of thepresent invention and to use it as filters or catalyst carriers,materials which form a number of pores in a molded product by burningout or gasifying by firing treatment, i.e., pore-forming agents may beadded. Examples of such pore-forming agents include organic spheres suchas foamed styrol beads, foamed urethane foam beads and polyethylenebeads; and organic fibers such as synthetic fibers and natural fiberssuch as hemp yarns and cottons. The amount of the spherical or fibrousorganic material which is the pore-forming agents varies depending upontheir uses, porosity and pore size. For example, the amount of thespherical or fibrous organic material added is from 5 to 100 parts byweight based on 100 parts by weight of the total amount of the zirconiapowder and zirconia fiber added. If the amount of the spherical orfibrous organic material is less than the lower limit, the porosity willbe insufficient. If the amount of the spherical or fibrous organicmaterial is more than the upper limit, the strength of the resultingzirconia composite refractory will be lowered and thus a sufficienthandling strength cannot be obtained.

Conventional refractory powders such as alumina, zirconia and silica canbe incorporated in the zirconia heating element according to the presentinvention to form a composite material. Further, various auxiliaries canbe incorporated in the present zirconia heating element in order toimpart various performances to the heating element which is a finalproduct.

(5) Production

The zirconia heating element of the present invention can be preparedand molded by various methods.

Processes in which a zirconia fiber can be used as a reinforcing agentin embodiment having a zirconia powder incorporated include moldingprocesses such as isostatic pressing, hot pressing, uniaxial pressing,casting and injection molding.

In the present invention, the resulting molded product may be driedafter molding. The molded product may or may not be subjected to firingtreatment. Such treatments can be suitably selected depending upon thekind of components from which the zirconia refractory heating element isproduced, and the uses of the zirconia refractory heating element Forexample, the zirconia molded product may be dried after molding and maybe used as a heating element for passing a current or as a heatingelement for induction heating as it is. When gas components generatedfrom the added binder and firing shrinkage pose problems, firingtreatment can be applied When firing treatment is applied, the firingtemperature is, for example, from 800° to 2,200° C., and preferably from1,500° to 2,000° C. If the firing temperature is less than the lowerlimit, the sintered strength will be weak. Firing can also be carriedout by directly passing a current through the heating element or byconducting induction heating of said heating element using means such ascoils for induction heating.

Firing of the resulting zirconia heating element can be carried out bycutting or dividing into a desired shape.

In the case of molded products essentially consisting of zirconiafibers, molding can be carried out by various procedures depending uponthe form of the desired final products. For example, in the case ofplate-shaped molded products, molding can be carried out by a lay-upmolding method A binder which is the crystal stabilizer described aboveis optionally added to a predetermined amount of the zirconia fibers andthe resulting mixture is dispersed in a dispersion medium to prepare aslurry. The slurry is subjected to lay-up molding to obtain aplate-shaped molded product. In lay-up molding, it is desirable that thebinder is not substantially dissolved in the dispersion medium This isbecause an effect of preventing migration phenomenon cannot be obtainedwhen the binder is dissolved in the dispersion medium. Accordingly, inthe cases of nonaqueous metal salts and oxides, water can be used as adispersion medium. In the cases of aqueous metal salts, it is desirablethat kerosene, heavy oil, alcohols and the like be used as a dispersionmedium. Sizing agents such as polyvinyl acetate, polyacrylic acid,carboxymethyl cellulose, methyl cellulose and polyethylene oxide may beadded to a dispersion medium for purposes of imparting viscosity to thedispersion and providing strength after drying.

Molding methods other than lay-up molding include a method wherein adispersion medium containing a sizing agent is sprayed to an assemblyhaving a predetermined shape composed of zirconia fibers; and anothermethod wherein the assembly is immersed in a dispersion mediumcontaining a sizing agent.

Thereafter, the molded product is usually subjected to drying treatment.Drying can be carried out by various methods such as spontaneous drying,blower drying, hot air drying and heat ray drying.

The zirconia fiber molded product according to the present invention canbe impregnated with a zirconia sol and/or a solution of a zirconiumcompound which can convert into zirconia by firing, before or afterdrying, or before or after firing. Firing carried out after impregnationrenders the connection structure of fibers to a structure ideal as aheating element.

The term "a zirconia sol and/or a solution of a zirconium compound whichcan convert into zirconia by firing" is already described and thereforeomitted herein.

The zirconia fiber molded product according to the present invention canbe used as a heating element for passing a current or as a heatingelement for induction heating after drying The molded product may beutilized in such uses after firing. By firing, the zirconia sol and/orthe solution of the zirconium compound convert into zirconia, and thebinder which is the crystal stabilizer contributes to bonding betweenzirconia fibers and stabilization of crystals by the zirconia fibers. Itis desirable that the firing temperature varies suitably depending uponthe kind of the added binder and the like, for example, from 1,300° C.to 2,000° C. The starting composition is heated to a firing temperatureand maintained at this temperature for a predetermined period of time toobtain a zirconia fiber molded product.

The zirconia refractory heating element according to the presentinvention can be formed in the various forms or dimensions. However, itis, in general, required that the diameters of the end or terminal partsin the form of a rod are larger than that of the heating part to reducethe resistance and heating temperature of the former compared with thoseof the latter.

The zirconia refractory heating elements according to the presentinvention are described in detail by Examples 1 through 6 andComparative Examples 1 through 4 with reference to FIGS. 1-4. Examples 1through 4 and Comparative Examples 1 and 2 are examples whereincurrent-passing heating is carried out, and Examples 5 and 6 andComparative Examples 3 and 4 are examples wherein induction heating iscarried out.

[B] Production of Laminate-shaped Zirconia Refractory Heating Elements

In particular, the zirconia refractory heating elements described abovecan be prepared in the form of a laminate to use the laminate with goodresults. Thus, the present invention provides also a process forproducing such a laminate-shaped zirconia refractory heating element. Inthis case, there is used a starting material obtained by incorporating aflexible binder in a filler composed of a zirconia fiber and zirconiapowder to which a zirconia stabilizer has been added. The startingmaterial is spread to form a refractory sheet having flexibility at roomtemperature and a high strength.

The refractory sheet is then wound in the form of a roll to form anapproximately cylindrical laminate. The refractory sheet is folded onceor zigzag folded many times to stack it to form a multilayer-shapedlaminate. Alternatively, refractory sheets previously cut to apredetermined size are stacked to form a multilayer-shaped laminate.

The laminate is fired to cure it in a state having the shape describedabove, thereby obtaining a zirconia refractory heating element of thepresent invention in the form of a laminated molded product.

In this heating element, the zirconia fibers have excellent mechanicalstrength and thermal shock properties and therefore they remarkablyenhance the mechanical characteristics of the heating element and act asa reinforcing agent for zirconia powder. Further, the zirconia fibershave a high melting point of at least about 2,600° C. and thereforephenomena such as decomposition and melting which often occur in thecases of refractory fibers other than the zirconia fibers do not occur.Because the present heating element has a structure wherein the heatingelement sheet is wound and laminated in the form of a roll or laminatedin the form of a multilayer, damage of only one layer of the heatingelement can occur if the heating element should be damaged. Thus, as awhole, variation of characteristics such as electrical resistance of theheating element is substantially inhibited.

The heating element raises the temperature by placing it in a magneticfield generated by a high frequency current passed through a coil forinduction heating or by directly passing a current through the heatingelement.

In laminating the refractory sheet, the attaching side of a lead wirefor passing a current is fixed to the refractory sheet, thereafter saidrefractory sheet is laminated as described above, and the laminate isfired, whereby a heating element having the lead wire for passing thecurrent is obtained.

When a current is passed through the lead wire for passing the current,a current is directly passed through the heating element and the heatingelement generates heat by Joule heat due to the resistance of saidheating element.

In a process for producing a laminate-shaped zirconia refractory heatingelement according to the present invention, a refractory sheet havingflexibility at room temperature is first prepared.

The refractory sheet is obtained by incorporating from 20 to 50 parts byweight of a flexible binder in 100 parts by weight of a filler composedof from 5 to 80 parts by weight of a zirconia fiber and from 20 to 95parts by weight of a zirconia stabilizer-added zirconia powder.

The zirconia fiber, zirconia powder and zirconia stabilizer are asdescribed above.

As described above, the amount of the zirconia fiber added is also from5 to 80 parts by weight based on from 20 to 95 parts by weight of thezirconia powder. The preferred amount of the zirconia fiber added isfrom 10 to 50 parts by weight. If the amount of the fiber is less than 5parts by weight, the reinforcing effect will be little. If the amount offiber is more than 80 parts by weight, forming into a refractory sheetwill become difficult.

The flexible binder for use herein comprises an emulsion of a suitablesynthetic resin such as a vinyl acetate resin or polyacrylate and atleast one plasticizer such as ethylene glycol, glycerin or dibutylphthalate.

It is preferred that the amount of the plasticizer added be from 5 to 25parts by weight based on 100 parts by weight of the synthetic resinemulsion described above.

It is preferred that the amount of such a flexible binder added be from20 to 50 parts by weight based on 100 parts by weight of the fillerdescribed above. If the amount of the flexible binder added is less than20 parts by weight, the flexibility of the refractory sheet will beinsufficient and cracks are liable to occur in laminating the refractorysheet. If the amount of the flexible binder added is more than 50 partsby weight, the refractory heating element will become brittle after theflexible binder is burnt out.

A reinforcing binder can be optionally added to the refractory sheet. Apreferred reinforcing binder is a sol or aqueous solution which convertsinto zirconia or an analogous compound by firing. Examples ofreinforcing binders include aqueous solutions of water-soluble saltssuch as zirconyl acetate, zirconium acetate, yttrium acetate, zirconiumchloride and yttrium chloride, and zirconia sols. The reinforcing binderis used in an amount of from about 0 to about 30 parts by weight.

The use of the reinforcing binder provides a self-hardening refractorysheet and increases the strength. Accordingly, the drying operation andfiring operation of the refractory sheet are improved.

A refractory sheet having flexibility at room temperature is obtained byspreading the starting material for refractory sheet having such acomposition by means of rolls to form a sheet or by extruding thestarting material by extrusion molding to form a sheet. It is preferredthat the thickness of the refractory sheet be from about 0 1 to about 1millimeter.

An approximately cylindrical laminate can be then formed by winding therefractory sheet around a fine rod-like core, or by winding therefractory sheet such that one end of said sheet is a base without usingthe core. When metal pipes and plastic pipes having the requireddiameter are used as the core, a cylindrical laminate can be formed.While the core described above is removed by withdrawing it after dryingor by burning out it during firing in the case of wood or plastic, thecore serves to prevent the refractory from deforming during drying andplays a role of preventing overheat of the central portion of a heatingelement by forming a through hole in the refractory sheet after removingthe core.

Alternatively, a multilayer-shaped laminate can be formed by folding therefractory sheet once or by zigzag folding it many times to stack it.Refractory sheets previously cut to a predetermined size may be stacked.

The wound end of the laminate described above may be secured to a lowerlayer by pressing. Alternatively, the wound end of the laminate may befixed by applying a mortar (a zirconia curing composition) as describedhereinafter.

In forming the laminate as described above by the refractory sheet(s), alead wire for passing a current can be attached. In the cases ofcylindrical solid or hollow laminates, the following methods can beused. In one method, a lead wire is securely fixed to the leadwire-attaching portion of a refractory sheet in such a state that theend of the lead wire is bent shortly to insert in the refractory sheet,and the refractory sheet is wound and laminated as it is. In anothermethod, a refractory sheet is cut to prepare lead wire-attaching sheetshaving a narrower width and the end of a lead wire is inserted in thelead wire-attaching portion of the lead wire-attaching sheets tosecurely fix the lead wire to the sheet. In a further method, a leadwire is wound around a lead wire-attaching portion of a refractory sheetto fix the lead wire to the refractory sheet, and the leadwire-attaching sheet is wound and laminated to both ends of a laminatebody to attach the lead wire.

In the case of a multilayer-shaped laminate, the end of a short bentlead wire is inserted in the lead wire-attaching portion of a refractorysheet to securely fix the lead wire to the refractory sheet and thus therefractory sheet is folded in the form a multilayer to laminate it.

The zirconia curing composition described in detail hereinafter may beapplied to the attaching site of the lead wire described above and thelead wire-attaching sheet to more strongly adhere them.

Besides the method of applying the curing composition to the winding endof the laminate as described above, the curing composition may be usedto strongly adhere stacked layers by applying it to the whole surface ofa refractory sheet and then forming it into a laminate.

A zirconia refractory heating element according to the present inventionis obtained by firing the resulting laminate after drying treatment. Thefiring temperature can be, for example, at least 800° C. The firingtemperature is preferably at least 1,400° C., more preferably from1,500° to 2,400° C. if the firing temperature is below the lower limit,the sintered strength will be weak. Firing can also be carried out bysubjecting the laminate to induction heating after drying treatment.

In order to prevent the deformation of the laminate during firing, thefollowing methods can be used. In one method, a hole is pierced in oneend of said laminate, and an alumina pipe is passed through the hole tosuspend the laminate. In another method, the outside of the laminate isprotected by a holding fixture to hold the shape and firing treatment iscarried out in this state. In these cases, cores which can be burnt outmay be used.

When the firing temperature exceeds 1,700° C., it is difficult to fire alaminate having a previously attached platinum lead wire or a suspendedlaminate, in respect of strength. Thus, in these cases, it is preferredthat the following method be used. A laminate is first subjected toprefiring at a temperature of from 1,400° to 1,700° C. to obtainstrength sufficient to place the laminate in a furnace as a heatingelement, the prefired laminate is attached to a furnace, and a currentis passed through the laminate or the laminate subjected to inductionheating to fire it.

Attachment of a lead wire for passing a current may be carried out afterfiring.

The process for producing the zirconia heating element in the form oflaminated molded products is described in more detail by Examples 7, 8and 9 and Comparative Examples 5 through 7 with reference to FIGS. 5 and6.

C] Zirconia Refractory Heating Structure and Process for Producing theSame

The present invention also relates to a zirconia heating structure and aprocess for producing the same.

The zirconia heating structure according to the present invention is onewhich generates heat by passing a current, said structure comprising thezirconia heating element as described above and a lead member forpassing a current, wherein the junction portions between said zirconiaheating element and said lead member for passing the current and theperipheries thereof are coated with a zirconia curing compositioncomprising a zirconia fiber, a zirconia powder, a water-solublezirconium salt (its aqueous solution exhibiting an acidity) and yttriaor a water-insoluble yttrium compound to join the zirconia heatingelement and the lead member. In producing the structure, the zirconiacuring composition described above may be coated on the membersdescribed above and cured at room temperature. Alternatively, after thezirconia curing composition is cured and fired, the cured compositionmay be impregnated with a zirconia sol and/or a solution of a zirconiumcompound.

As described above, the zirconia heating element used in the zirconiaheating structure of the present invention can use the following moldedproducts: a molded product obtained by suitably mixing a zirconiapowder, an additive, a stabilizer, a zirconia fiber and the like, hot orcold pressing the mixture and firing it; a molded product obtained byslip casting a similar mixture and firing it; or a molded productobtained by molding a zirconia fiber alone or a mixture of a zirconiafiber and a zirconia powder by a lay-up or pouring method and firing theresulting molded matter. It is desirable that a zirconia fiber becontained in these zirconia heating elements from the standpoint ofutility. The zirconia fiber may contain oxides such as MgO, CaO, Y₂ O₃and Gd₂ O₃ which act as stabilizers for zirconia.

The zirconia curing composition for securing the zirconia heatingelement body and current-passing lead member which are used in thepresent invention include a zirconia curing composition exhibitingself-hardening properties at room temperature which is obtained by usinga zirconia fiber, a zirconia powder, a water-soluble zirconium salt as abinder (its aqueous solution exhibiting an acidity) and yttria or awater-insoluble yttrium compound as a curing agent.

The zirconia fiber used in the zirconia curing composition has excellentmechanical strength and thermal shock properties and therefore thezirconia fiber acts as a reinforcing agent for the zirconia powder. Thezirconia fiber absorbs stress resulting from the difference in thermalexpansion between the heating element body and the lead member forpassing the current, and its conductivity is improved.

Further, this composition exhibits self-hardening properties at roomtemperature due to the water-soluble zirconium salt which is the binderand the water-insoluble yttrium compound which is the curing agent, andthe composition strongly joins the zirconia heating element and the leadmember for passing the current.

The zirconia fiber and zirconia powder used in the zirconia curingcomposition are as described above. The water-soluble zirconium salts(their aqueous solution exhibiting an acidity) for use herein includezirconium acetate, zirconyl acetate, zirconium oxychloride, zirconiumnitrate and zirconium sulfate. Taking into account the drawbacks thattoxic gases are generated by decomposition of the binder component whenthe molded product obtained by curing at room temperature is fired, itis preferred that zirconium acetate or zirconyl acetate be used.

It is necessary that the concentration of the aqueous solution be atleast 5%. If the concentration of the aqueous solution is less than 5%,the strength obtained after curing will be insufficient and handling ofcured products will become difficult.

Another essential component of said zirconia curing composition isyttria or a water-insoluble yttrium compound. It is necessary thatyttria has a purity of at least 90% by weight It is desirable that thegrain size of yttria be no more than 1 millimeter. If the purity ofyttria is less than 90% by weight or if the grain size of yttria is morethan 1 millimeter, it is difficult for yttria to act as the curingagent.

The water-insoluble yttrium compounds include yttrium carbonate andyttrium hydroxide. These water-insoluble yttrium compounds are usuallymanufactured and sold in the form of a fine powder with a grain size ofno more than 0.1 millimeter. While it is unnecessary to take intoaccount the grain size, it is necessary that the purity of yttria formedby heating be at least 90% by weight.

The weight ratio of yttria or the water-insoluble yttrium compound tothe water-soluble zirconium salt is as follows: The weight ratio ofyttria (when the water-insoluble yttrium compound is used, the termrefers to yttria formed by heating) to zirconia formed from thewater-soluble zirconium salt is preferably 0.5-5:1. The curing time canbe adjusted within the range of from 10 minutes to 10 hours by adjustingthe ratio within the range described above. If the weight ratio is lessthan 0.5:1, the curing time will be excessively increased and thereforesuch a weight ratio is impractical If the weight ratio is more than 5:1,the curing time will be excessively decreased and the zirconia curingcomposition will cure during kneading.

The amount of the aqueous solution of the zirconium salt added is withinthe range of from 10 to 60 parts by weight based on 100 parts by weightof the total amount of the zirconia fiber and the zirconia powder. Theamount of the aqueous solution of the added zirconium salt can varydepending upon the desired consistency of the body.

When viscosity modification is necessary in using the zirconia curingcomposition, conventional water-soluble organic sizing agents such aspolyvinyl alcohol, methyl cellulose, carboxymethyl cellulose andpolyethylene oxide can also be added.

It is necessary that the lead member for passing the current hassufficient conductivity at room temperature, that the lead member doesnot denature at considerably high temperatures and that its conductivitydoes not decrease extremely. The known materials from which the leadmember for passing the current include platinum, molybdenum disilicide,silicon carbide and lanthanum chromite. These materials can be usedherein. When a platinum wire rod is used as the lead member for passingthe current, it is a material suitable as the current-passing leadmember for the zirconia heating element unless the platinum wire rod isexposed to a temperature of 1 770° C. which is the melting point ofplatinum, or above.

In the zirconia heating structure of the present invention, the surfaceof the zirconia heating element may be in direct contact with thesurface of the lead member for passing the current. Both members may beelectrically connected via the zirconia curing composition describedabove. Further, the lead member for passing the current and the zirconiaheating element may be physically preset (e.g., threaded, meshed bygrooves, or wound) before the zirconia curing composition is used.Further, concavities and convexities can be formed in the lead memberfor passing the current and the zirconia heating element by fluting orthe like so that (a) the cured product of the zirconia curingcomposition and the lead member, and (b) the cured product of thezirconia curing composition and the zirconia heating element arephysically set after the zirconia curing composition is cured.

The zirconia heating structure of the present invention may or may notbe fired. Further, the zirconia heating structure of the presentinvention may be fired and thereafter impregnated with a zirconia sol oraqueous solution of a zirconium compound which converts into zirconia byfiring. Thereby, the pores of the zirconia cured composition are packedand such a treatment renders the texture more dense, enhances thestrength and decreases the electrical resistance to improve theconductivity.

Firing treatment is preferably carried out at a temperature of from1,200° to 1,750° C. (i.e., from the decomposition temperature of thebinder component of the zirconia curing composition to theheat-resistant temperature of the lead member for passing the current).

The impregnating solutions include a zirconia sol and/or an aqueoussolution of a zirconium compound which can convert into zirconia byfiring. In a preferred embodiment of the present invention, the zirconiasol is a milky colloidal solution wherein zirconia having a particlediameter of, for example, about 70 millimicrons is suspended in water.For example, an aqueous solution of a zirconium salt can be used as theaqueous solution of the zirconium compound. Examples of the zirconiumsalts include zirconyl acetate, zirconyl nitrate, zirconyl oxychloride,zirconyl sulfate, and ammonium zirconyl carbonate. Hydrolyzates ofzirconium alkoxides such as tetrapropyl zirconate and tetrabutylzirconate can be used as the aqueous solution of the zirconium compound.These are converted into zirconia by firing them. The impregnatingsolution can be incorporated in a fired body by spraying it onto thesurface of the fired body or by immersing the fired body in theimpregnating solution.

In the present invention, other impregnating auxiliary agents can alsobe added. For example, chlorides, sulfates and nitrates of magnesium,yttrium, calcium, samarium, cadmium, lanthanum and neodymium can also beadded as crystal stabilizers. The amount of the crystal stabilizer addedis from 0.5 to 30 parts by weight based on 100 parts by weight of theimpregnating solution (on a ZrO₂ basis).

The present zirconia heating structure and process therefor aredescribed in detail by Examples 11 through 14 and Comparative Examples 7and 8 with reference to FIGS. 8 through 11.

EFFECTS OF THE INVENTION

The zirconia refractory heating element according to the presentinvention generates heat by passing a current or by induction heating,and has the following effects and advantages.

(a) Because the zirconia fiber used in the zirconia refractory heatingelement according to the present invention has excellent mechanicalstrength, resistance to damage is high and the progress of cracks can beinhibited even if small flaws are present in the heating element.

(b) Because the zirconia refractory heating element according to thepresent invention contains the zirconia fiber having excellentmechanical and thermal characteristics, it imparts good thermal shockresistance to the heating element, and can be used in uses which are notpossible in the case of the prior zirconia refractory element. Forexample, the present zirconia refractory heating element can be used asa lining material for high-speed temperature elevation furnaces.

(c) When the heating element of the present invention is impregnatedwith the zirconia sol and/or the aqueous solution of the zirconiumcompound as a binder or the zirconia sol or the aqueous solution of thezirconium compound is added to the heating element of the presentinvention, bonding between zirconia fibers to each other or bondingbetween the zirconia powder and the zirconia fiber is increased afterfiring Further, the binder per se forms a tough coat and has heatresistance equivalent to that of the zirconia fiber Thus, the resultingheating element can exhibit high mechanical strength and excellentchemical stability.

(d) Because the heating element according to the present invention hasexcellent thermal shock resistance, the low heat-conductive propertiesof zirconia can be fully utilized to sufficiently exhibit theheat-insulating characteristics.

(e) Because the present heating element contains zirconia fibers havingexcellent mechanical characteristics, it exhibits flexibility, andfolding due to mechanical stress becomes difficult.

(f) In the heating element of the present invention, when the crystalstabilizer for zirconia or its precursor is used as the binder, thebinder strongly adheres to the surface of the zirconia fibersAccordingly, the binder is not transferred to the surface of the moldedproduct during drying and/or during sintering. Therefore, migrationphenomenon which occur in the case of colloidal silica does not occur,and the whole of the molded product can have a uniform composition andstrength. The metal oxide which is the crystal stabilizer has a highmelting Point of from 2.500° to 2,800° C. and therefore a zirconia fibermolded product having high refractory properties can be obtained due tothe combination with the zirconia fiber having a melting point of 2,600°C.

(g) The zirconia refractory heating element obtained by the presentinvention has excellent exothermic characteristics. The mechanicalstrength and thermal shock resistance are excellent becausereinforcement by the zirconia fibers is achieved. When the zirconiarefractory heating element has a laminated structure, the followingadvantages are obtained: if cracks should generate, the cracks arepresent in only one layer, and variation of characteristics of the wholeof the heating element, e.g., change of electric resistance value can beinhibited to a lesser extent. The present heating element can withstandlonger usage. Moreover, the zirconia refractory heating element can bereadily produced.

(h) According to the process of the present invention, the degree offreedom of the shape of the heating element is large. When the heatingelement is cut to a shape which allows for the variation after firing,it is unnecessary to carry out machining for finishing. The manufacturesteps and manufacture times can be reduced.

(i) Bubbles entrained in the materials in the operation of forming thematerials into the flexible sheet can be removed by the process of thepresent invention, and more homogeneous heating elements can be readilyobtained.

Further, according to the present zirconia heating structure and theprocess therefor, practical heating elements containing the zirconiafibers can join the lead member with the zirconia curing compositionobtained by incorporating the zirconia fibers by the process of thepresent invention. Further, the cured product of said composition hasccnductivity and therefore the area of the contact portion between theheating element body and the lead member for passing a current can beincreased. The damage of the heating element body attributable to thefact that a local current is passed through the contact portion can beprevented Heating structures for high temperatures having even higherutility can be provided.

If the pores of said cured composition are impregnated with the zirconiasol and/or the aqueous solution of the zirconium compound after firingthe curing composition, bonding of the zirconia fibers to each other orbonding between the zirconia powder and the zirconia fiber can beincreased and the electrical conductivity can be further improved.

While Examples described above demonstrate such embodiments that theheating element body of the zirconia heating structure of the presentinvention contains the zirconia fibers, heating element bodies used inthe present invention are not limited thereto, and zirconia heatingelements which have been used in the prior art can also be used herein.

EXAMPLE 1

Fifty parts by weight of a magnesia-stabilized zirconia powder having anaverage diameter of from 1 to 0.3 millimeter (5% of MgO and 95% ofZrO₂), 50 parts by weight of a magnesia-stabilized zirconia powderhaving a grain size of no more than 0.3 millimeter (5% of MgO and 95% ofZrO₂), 100 parts by weight of a magnesia-added zirconia fiber having anaverage diameter of 5 micrometers and an average length of from 20 to 30millimeters (manufactured by Shinagawa Shirorenga), 1 part by weight ofmethyl cellulose and 5 parts by weight of an aqueous zirconium acetatesolution (ZrO₂ =15%) were blended. After mixing, the blend wasuniaxially pressed under a pressure of 50 Kg/cm² and dried to obtain azirconia heating element. The zirconia heating element was fired at atemperature of 1,800° C. to obtain a fired product The characteristicsof the fired product are shown in Table 1.

EXAMPLE 2

One hundred parts by weight of an yttria-stabilized zirconia powderhaving an average diameter of 1 millimeter (7% of Y₂ O₃ and 93% ofZrO₂), 50 parts by weight of an yttria-stabilized zirconia fiber havingan average diameter of 5 micrometers and an average length of from 20 to30 millimeters (manufactured by Shinagawa Shirorenga), 5 parts by weightof a zirconium chloride powder, 2 parts by weight of foamed styrol beadshaving an average diameter of 1 millimeter and 30 parts by weight ofwater were blended. After mixing, the blend was poured into a moldingflask and dried to obtain a zirconia heat-insulating refractory. Thisrefractory was fired at a temperature of 1,800° C. to obtain a firedproduct. The characteristics of the fired product are shown in Table 1.

COMPARATIVE EXAMPLE 1

A zirconia molded product was obtained and fired as in example 1 exceptthat no zirconia fibers were blended. The characteristics of theresulting product are shown in Table 1.

Cracks did not occur after drying treatment used in the productionprocess.

COMPARATIVE EXAMPLE 2

A zirconia heating element was obtained as in Example 2 except that nozirconia fibers were blended.

Several cracks occurred after drying treatment used in the productionprocess. The crack-free portions were fired. The characteristics of thefired product are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                                   Compar-  Compar-                                                              ative    ative                                                Example                                                                              Example  Example  Example                                              1      2        1        2                                         ______________________________________                                        Cracks after drying                                                                        none     none     none   several                                                                       cracks                                  Apparent porosity %                                                                        45       85       40     75                                      Bulk specific gravity                                                                      2.3      1.4      2.4    1.6                                     Bending strength                                                                           50       20       35     8                                       Kg/cm.sup.2                                                                   Deflection at bending                                                                      5        3        0.5    0.3                                     rupture mm                                                                    Thermal shock                                                                              normal   normal   crack at                                                                             crack at                                resistance                     a first                                                                              a first                                                                time   time                                    ______________________________________                                         Note 1) The shape of a specimen: 40 × 40 × 160 millimeters;       span distance: 100 millimeters; and threepoint bending test.                  Note 2) Thermal shock resistance: spalling test (room temperature             1,500° C. for 30 minutes  room temperature); the shape of a            specimen: 230 × 114 × 65 millimeters; the test was repeated 5     times.                                                                   

EXAMPLE 3

The molded product of Example 1 was immersed in an aqueous zirconiumacetate solution (ZrO₂ =15%), pulled up, dried for 24 hours at atemperature of 100° C., and then heated at a temperature of 1,500° C. toobtain a zirconia heating element.

EXAMPLE 4

Fifty parts by weight of an yttria-stabilized zirconia fiber having anaverage diameter of 5 micrometers and an average length of from 20 to 30millimeters, 50 parts by weight of a magnesia-added zirconia fiberhaving an average diameter of 5 micrometers and an average length offrom 20 to 30 millimeters and 5 parts by weight of an yttria powderhaving an average grain size of from 10 to 50 micrometers were added towater as a dispersion medium to prepare a suspension for lay-up.

This suspension was laid to form a plate-shaped molded product. Aftermolding, the molded product was dried for 24 hours at a temperature of100° C. and then fired at a temperature of 1,600° C. to prepare a fibermolded product. The resulting molded product was immersed in an aqueouszirconium acetate solution (zirconia yield of 15%), pulled up, dried for24 hours at a temperature of 100° C. and then heat treated at atemperature of 1,000° C. to obtain a zirconia refractory heatingelement.

Service Test (Current-passing Heating)

The materials and production conditions as described in Examples 1through 4 and Comparative Examples 1 and 2 were used to prepare heatingelements 1 wherein both ends 1a and 1a are larger than the centralportion as shown in FIG. 1. The heating elements 1 were preheated to1,500° C. by means of an auxiliary heater, a current is passed througheach heating element 1 to generate heat to increase the temperature upto 2,000° C. in 30 minutes. During this period, the electric resistanceand repetition service properties were measured. (Dimension of heatingelements: total length: 200 millimeters; longitudinal length of centralsmaller portions except both ends 1a: 150 millimeters; length of oneperipheral side of smaller portions: 10 millimeters; and length of oneside of a tip surface of both ends 1a: 20 millimeters.)

The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Example Service Properties     Electric                                       No.     (Repetitive Heat Generation Number)                                                                  Resistance                                     ______________________________________                                        Example 1                                                                             failure after heating 18 times                                                                       --                                             Example 2                                                                             failure after heating 20 times                                                                       --                                             Example 3                                                                             normal after heating 50 times                                                                        --                                             Example 4                                                                             normal after heating 100 times                                                                       2                                              Compara-                                                                              failure after heating once                                                                           3                                              tive                                                                          Example 1                                                                     Compara-                                                                              failure after heating once                                                                           4                                              tive                                                                          Example 2                                                                     ______________________________________                                    

Note) Electric resistance is an electric resistance (Ω·cm) when 2,000°C. is reached. This electric resistance was calculated fromcurrent-voltage values which were supplied to the heating elements whencurrent-passing heating was carried out.

Cylindrical or tubular heating elements having both ends 2a and 2ahaving a diameter larger than that of the central portion as shown inFIG. Z were subjected to the same test as described above, approximatelythe same results were obtained. (Dimension of the heating elements:total length: 200 millimeters; length of a central smaller portionexcept both ends 2a: 100 millimeters: diameter of the smaller portion:10 millimeters; and diameter of both ends 2a: 20 millimeters.)

EXAMPLE 5

Fifty parts by weight of a magnesia-stabilized zirconia powder having aparticle diameter of from 1 to 0.3 millimeter (5% of MgO and 95% ofZrO₂), 50 parts by weight of a magnesia-stabilized zirconia powderhaving a grain size of no more than 0.3 millimeter (5% of MgO and 95% ofZrO₂), 100 parts by weight of a magnesia-added zirconia fiber having anaverage diameter of 5 micrometers and an average length of from 20 to 30millimeters (manufactured by Shinagawa Shirorenga), 2 parts by weight ofmethyl cellulose, 12 parts by weight of an aqueous zirconium acetatesolution (ZrO₂ =4%) and 3 parts by weight of ethylene glycol as aplasticizer were blended. This blend was mixed and thereafter formedinto a plate-shaped molded product. This refractory was dried andthereafter fired at a temperature of 1,800° C. to obtain a firedproduct. The characteristics of the fired product are shown in Table 3.

A green body obtained by injection molding the blend after mixing wasdried, fired at a temperature of 1,800° C. to obtain a tubular zirconiaheating element.

This heating element was subjected to a service test (induction heating)described hereinafter.

EXAMPLE 6

One hundred parts by weight of an yttria-stabilized zirconia powderhaving a particle diameter of no more than 0.3 millimeter (7% of Y₂ O₃and 93% of ZrO₂), 50 parts by weight of an yttria-stabilized zirconiafiber having an average diameter of 5 micrometers and an average lengthof from 20 to 30 millimeters (manufactured by Shinagawa Shirorenga), 1part by weight of foamed styrol beads having an average diameter of 1millimeter, 30 parts by weight of water and 2 parts by weight of yttriawere blended and thereafter further 30 parts by weight of water wereadded to this blend. The resulting blend was mixed and formed into aplate-shaped molded product. This refractory was dried and thereafterfired at a temperature of 1,800° C. to obtain a fired product. Thecharacteristics of the fired product are shown in Table 3.

The mixture described above was mixed and thereafter poured into amolding flask The mixture was dried and thereafter fired at atemperature of 1,800° C. to obtain a fired product. This fired productis slightly machined to obtain a tubular zirconia heating element. Thisheating element was subjected to the service test as described inExample 5.

COMPARATIVE EXAMPLE 3

A plate-shaped fired product and a zirconium heating element wereobtained as in Example 5 except that no zirconia fibers were blended.The characteristics of this fired product are shown in Table 3.

Cracks did not occur after drying treatment used in the productionprocess.

COMPARATIVE EXAMPLE 4

A specimen was obtained by firing as in Example 6 except that nozirconia fibers were blended. However, in this procedure, one sample per2-3 samples exhibited cracks after firing, and therefore could not beused as a specimen.

                  TABLE 3                                                         ______________________________________                                                                   Compar-  Compar-                                                              ative    ative                                                Example                                                                              Example  Example  Example                                              5      6        3        4                                         ______________________________________                                        Cracks after drying                                                                        none     none     none   several                                                                       cracks                                  Apparent porosity %                                                                        41       85       39     75                                      Bulk specific gravity                                                                      2.7      1.5      2.8    1.6                                     Bending strength                                                                           90       20       95     7                                       Kg/cm.sup.2                                                                   ______________________________________                                         Note 1) Shape of specimen: 40 × 40 × 160 millimeters; span        distance: 100 millimeters; and three point bending test.                 

SERVICE TEST (INDUCTION HEATING)

Each of tubular heating elements 3 was prepared as described in Examples5 and 6 and Comparative Examples 3 and 4 (FIGS. 3 and 4). As shown inFIGS. 3 and 4, an appropriate heat-insulating material was applied tothe surface of the heating element 3 to form a heat-insulating layer 4.This was preheated to 1,500° C. by means of an auxiliary heater andthereafter placed in a magnetic field generated by passing a highfrequency current through a coil for induction heating 5 to generateheat to increase the temperature up to 2,200° C. in 30 minutes. Duringthis period, repetitive service properties were measured (Dimension ofthe heating element: diameter: 100 millimeters; thickness: 4millimeters; and length: 100 millimeters). A zirconia (ZrO₂)heat-insulating board was used as a support for a heating element.

The results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Example Service Properties     Electric                                       No.     (Repetitive Heat Generation Number)                                                                  Resistance                                     ______________________________________                                        Example 5                                                                             failure after heating 20 times                                                                       --                                             Example 6                                                                             failure after heating 30 times                                                                       --                                             Compara-                                                                              failure after heating once                                                                           --                                             tive                                                                          Example 3                                                                     Compara-                                                                              failure after heating once                                                                           --                                             tive                                                                          Example 4                                                                     ______________________________________                                    

It is not necessarily required to provide the heat-insulating layer 4described above.

The values such as those of dimensions shown in Examples are exemplaryvalues and it is possible to vary them appropriately. The shape of theheating element is not limited to the shape used in Examples describedabove and the heating element can be formed in any desired form.Naturally, it is possible to use the heating element (1, and 2) shown inFIGS. 1 and 2 in an induction heating process and it is possible to usethe heating element (3) shown in FIGS. 3 and 4 in the current-passingheating process.

EXAMPLE 7

Thirty three parts by weight of the total amount of a polyvinyl acetateemulsion and a plasticizer (dibutyl phthalate) as flexible binders wereadded to 50 parts by weight of an yttria-stabilized zirconia fiber (Y7ZFiber available from Shinagawa Shirorenga) and 50 parts by weight of anyttria-stabilized zirconia powder (7% of Y₂ O₃ and 92% of ZrO₂). Theblend was thoroughly mixed and transferred to a planar p? ate. The blendwas spread using rollers to prepare a flexible sheet 6 having athickness of 0.5 millimeters (See FIG. 5). This flexible sheet 6 wasthen wound around a stainless rod 7 having a diameter of 2 millimetersas a core to form a laminate body 8. On the other hand, a flexible sheetwas cut to dimension which allows for variation due to firing, therebypreparing a sheet 6a for attaching a lead wire. This sheet 6a was woundaround both ends of the laminate body 8 in the form of layers whilesecurely fixing it by inserting the bent tips of lead wires 9 in thesheet 6a for attaching lead wires as shown in FIG. 5(a). Drying wascarried out for 10 hours at a temperature of 150° C. as it was.Thereafter, the stainless rod 7 was removed, and prefiring was carriedout at a temperature of 1,500° C. Thereafter, the prefired product wasplaced in an electric furnace and firing was carried out at atemperature of 2,000° C. by passing a current. The resulting firedproduct was used. The results are shown in Table 5.

EXAMPLE 8

The flexible binder used in Example 7 was incorporated in 50 parts byweight of the yttria-stabilized zirconia fiber used in Example 7, 45parts by weight of an yttria-stabilized zirconia powder and 5 parts byweight of a magnesia-stabilized zirconia powder (4% of MgO and 96% ofZrO₂), and a flexible sheet was prepared as in Example 7. As shown inFIG. 6(a), this sheet was cut so that a concavity 10 having a taper wasformed at its one end, thereby forming sheet portions 4b for attachinglead wires at both ends. The flexible sheet 6 was windingly laminated onthe stainless rod 7 in the cylindrical form starting from the bodyportion, and thereafter the sheet portions 4b were wound around bothends of the laminate body 8 in the form of layers having tapers in aninner direction while attaching lead wires 9 as in Example 7. The wholewas prefired at a temperature of 1,600° C. Thereafter, the prefiredproduct was immersed in an aqueous solution containing zirconium acetateand yttrium acetate, and dried. The prefired product was placed in anelectric furnace, and preheated up to 1,500° C. at an elevation rate of3° C./minute by means of the electric furnace. Thereafter, a current waspassed through the laminate body until the temperature was 2,000° C. Thepreheated product was therefore fired. The resulting fired product wasused. The results are shown in Table 5.

EXAMPLE 9

Twenty eight parts by weight of a polyvinyl acetate emulsion containing3% of yttria and 18% of a plasticizer (ethylene glycol) and 8 parts byweight of an aqueous zirconyl acetate solution (concentration of 20% ona ZrO₂ basis) were incorporated in 20 parts by weight of theyttria-stabilized fiber used in Example 7, 65 parts by weight of anyttria-stabilized zirconia powder and 15 parts by weight of anunstabilized zirconia powder. The blend was mixed to prepare a flexiblesheet having a thickness of 1 millimeter. This sheet was wound threetimes around a paper pipe having a diameter of 100 millimeters. Theself-hardening composition containing the zirconia fiber, i.e., thezirconia curing composition described above was applied to the lastportion wound and the initial wound portion such that the width of about2 millimeters was obtained, and the sheet was affixed to an adjacentlayer. The laminate was allowed to stand for 12 hours in the intactstate, and thereafter dried for 6 hours at a temperature of 150° C. todecrease flexibility As shown in FIG. 7, the laminate 11 was placed in azirconia brick holding fixture 12 and its outer periphery was retained.The laminate 11 was prefired at a temperature of 1,600° C. The resultingprefired product was placed in an induction furnace and preheated.Thereafter, the prefired product was subjected to induction heating atan elevation rate of 2.5° C./min until the temperature was 2,200° C.Thus, the prefired product was fired. The resulting fired product wasused. The results are shown in Table 5.

COMPARATIVE EXAMPLE 5

Five percent of a hemp yarn was incorporated in 95% of anyttria-stabilized zirconia powder. A flexible sheet was prepared usingthe binder and molding method of Example 7. An approximately cylindricallaminate was formed. This laminate generated several cracks in prefiringat 1,500° C. The resulting fired product was used. The results are shownin Table 5.

COMPARATIVE EXAMPLE 6

An attempt was made to form a laminate from a sheet of only anyttria-stabilized zirconia powder using the same binder and moldingmethod as described in Example 7. However, slight cracks were generatedin the stage of winding the sheet, and therefore the subsequent stepswere discontinued.

                                      TABLE 5                                     __________________________________________________________________________                                Comparative                                                                          Comparative                                         Example 7                                                                           Example 8                                                                           Example 9                                                                            Example 5                                                                            Example 6                                  __________________________________________________________________________    Shape type                                                                             cylindrical                                                                         cylindrical                                                                         cylindrical                                                                          cylindrical                                                                          Manufacture                                                                   was dis-                                                                      continued                                                                     on the way                                 Lead wire                                                                              Platinum                                                                            Platinum                                                                            none   Platinum                                                   wire  wire         wire                                                       (simul-                                                                             (simul-      (simul-                                                    taneous                                                                             taneous      taneous                                                    molding)                                                                            molding)     molding)                                          Heating method                                                                         Current-                                                                            Current-                                                                            Induction                                                                            Current-                                                   passing                                                                             passing      passing                                           Highest retention                                                                      2,000° C.                                                                    2,100° C.                                                                    2,000° C.                                                                     2,000° C.                                  temperature                                                                   Repetitive                                                                             65 times                                                                            45 times                                                                            30 times                                                                             Cracks                                            elevation number            generated                                         (Service (normal)                                                                            (normal)                                                                            (slight                                                                              once, and                                         properties)          cracks were                                                                          failure                                                                recognized)                                                                          occurred                                                                      during                                                                        second                                                                        elevation                                                                     process                                           __________________________________________________________________________

In Examples 7 and 8, the electric resistance at 2,000° C. was from 3 to7 ω·cm. In Comparative Example 5, the electric resistance was about 6Ω·cm. Small variations in voltage and current were recognized whencurrent was passed. It is believed that this is because cracks grow whencurrent is passed.

EXAMPLE 10

As shown in FIG. 8 (a) and (b), a rod-shaped zirconia heating element 13containing 75% by weight of zirconia fiber and a molybdenum disiliciderod 14 which is a lead member for passing a current were secured using azirconia curing composition 15 having the following composition:

    ______________________________________                                        Composition          Parts by weight                                          ______________________________________                                        Zirconia fiber       50                                                       Zirconia powder      50                                                       Aqueous zirconyl acetate solution                                                                  55                                                       (ZrO.sub.2 yield of 20%)                                                      Y.sub.2 O.sub.3 powder                                                                              4                                                       ______________________________________                                    

The zirconia curing composition was allowed to stand for 4 hours untilit self-hardened, dried for 24 hours at a temperature of 110° C.,calcined at a temperature of 1,400° C. to develop strength, therebyobtaining a zirconia heating structure of the present invention.

The resulting heating structure was then placed in an electric furnace,and one ®nd of the molybdenum disilicide rod was arranged at the outsideportion of the electric furnace. A conductor was clamped, the heatingstructure was preheated up to 1,500° C. A current was passed through theheating structure to use it as a heating element. This heating elementcan generate heat to increase the temperature to 2,000°-2,300° C. bypassing a current. The junction between the heating element and the leadmember is good and junction ruptures do not occur when current ispassed.

A heating structure provided with a molybdenum disilicide lead memberfor passing a current, and which has been dried for 24 hours at atemperature of 110° C. was calcined at a temperature of 800° C. andplaced in a furnace The heating structure was slowly preheated up to1,500° C., and thereafter, a current was passed to slowly heat theheating structure up to 2,000° C.

In the case of this procedure, a SiO₂ coat cannot be formed on themolybdenum disilicide rod portion which is present in the outsideportion of the furnace during calcination, and thus the step of removingthe SiO₂ coat before clamping could be omitted. The heating structuresshown in FIG. 8(a) and (b) exhibited substantially the same junctionperformance.

The zirconia curing composition used in this example was formed into a10 mm×10 mm×80 mm rod, and this rod was fired for 3 hours at atemperature of 1,700° C. to prepare a specimen. This specimen was usedto examine the electrical resistance at high temperatures. The specimenhad a conductivity of 7.7 Ω·cm at 1,700° C.

EXAMPLE 11

Both ends of a rod-shaped zirconia fiber heating element 13 produced bythe lay-up method were subjected to grooving. As shown in FIG. 9, aplatinum wire 16 having a diameter of 0.5 millimeter was wound andthereafter a zirconia curing composition 15 having the same compositionas that used in Example 10 except that it contained 30 parts by weightof a zirconia fiber and 70 parts by weight of a zirconia powder wasapplied so that a groove formed in the heating element described abovewas embedded. The whole was fired for 2 hours at a temperature of 1,500°C. Thereafter, a zirconium acetate solution (ZrO₂ yield of 15%) wassprayed and allowed to stand for 4 hours. The whole was then dried for24 hours at a temperature of 110° C. and calcined at a temperature of1,200° C. It was attached so that one end of the platinum wire waspresent in the outside portion of the furnace. Said platinum wire wasconnected to a conductor. The whole was preheated to 1,500° C., and acurrent began to be passed carefully to gradually heat it up to 2,000°C.

The zirconium curing composition used in this example was formed into a10 mm×10 mm×80 mm rod and this rod was fired for 3 hours at atemperature of 1,700° C. to prepare a specimen. This specimen was usedto examine the electrical resistance at a high temperature. The specimenhad a conductivity of 6.2 Ω·cm at 1,700° C.

While the heating element was heated to a temperature higher than themelting point of platinum from which the lead member was produced inthis example, the temperature of terminal portions could be reduced byincreasing the area of the terminal portions of the heating element,whereby melting of the lead member could be prevented.

EXAMPLE 12

Both ends of a rod-shaped zirconia fiber heating element 13 produced bya lay-up method were subjected to grooving. As shown in FIG. 10, aplatinum wire 16 was wound and thereafter a sleeve 17 composed of thesame material as that of the heating element body was covered. Azirconia curing composition 15 having the same composition as that usedin Example 10 except that it contained 70 parts by weight of a zirconiafiber and 30 parts by weight of a zirconia powder was applied so that agap between the heating element body and the sleeve was embedded. Thewhole was fired for 2 hours at a temperature of 1,500° C. andimpregnated with an aqueous zirconium acetate solution (ZrO₂ yield of7%) to obtain a zirconia heating structure of the present invention.

The zirconia curing composition used in this example was formed into a10 mm×10 mm×80 mm rod, and this rod was fired for 3 hours at atemperature of 1,700° C. to obtain a specimen. This specimen was used toexamine the electrical resistance at high temperatures. The specimen hada conductivity of 8.0 Ω·cm at 1,700° C.

EXAMPLE 13

As shown in FIG. 11(a) and (b), grooves and holes 18 were formed in bothends of a rod-shaped heating element 13 composed of a zirconia powderand a zirconia fiber containing 30 parts by weight of the zirconiafiber. A zirconia curing composition 15 having the same composition asthat used in Example 10 (except that it contained 15 parts by weight ofa zirconia fiber and 85 parts by weight of a zirconia powder and anaqueous zirconium acetate solution (ZrO₂ yield of 15%) was used) waspoured while vibrating the grooves and holes 18. Thereafter, apremachined silicon carbide rod 14 was immediately inserted. As shown inFIG. 11(c), a portion of excess composition was used to apply it to thesurface of the silicon carbide rod and the whole was allowed to standfor 4 hours. Thereafter, the whole was dried for 24 hours at atemperature of 110° C., and calcined at a temperature of 800° C. toobtain a zirconia heating structure of the present invention.

The zirconia curing composition used in this example was formed into a10 mm×10 mm×80 mm rod, and this rod was fired for 3 hours at atemperature of 1,700° C. to prepare a specimen. This specimen was usedto examine the electrical resistance at high temperatures. The specimenhad a conductivity of 5.5 Ω·cm at 1,700° C.

EXAMPLE 14

Example 11 was repeated except that the following procedure was used. Inorder to join a platinum wire to a heating element body, the followingzirconia curing composition was used and applied Thereafter, thezirconia curing composition was dried for 4 hours at a temperature of40° C., further dried for 24 hours at a temperature of 110° C., andcalcined at a temperature of 1,500° C. to obtain a zirconia heatingstructure of the present invention.

    ______________________________________                                        Composition          Parts by weight                                          ______________________________________                                        Zirconia fiber       30                                                       Zirconia powder      70                                                       Aqueous zirconyl acetate solution                                                                  55                                                       (ZrO.sub.2 yield of 15%)                                                      Y.sub.2 O.sub.3 powder                                                                              7                                                       ______________________________________                                    

The zirconia curing composition used in this example was formed into a10 mm×10 mm×100 mm rod, and this rod was fired for 2 hours at atemperature of 1,500° C. to prepare a specimen. This specimen was usedto examine the electrical resistance at a high temperature. The specimenhad a conductivity of 6 Ω·cm at 1,700° C.

COMPARATIVE EXAMPLE 7

    ______________________________________                                        Composition          Parts by weight                                          ______________________________________                                        Zirconia powder      100                                                      Aqueous zirconyl oxychloride                                                                        50                                                      solution (Zirconia yield of 20%)                                              Y.sub.2 O.sub.3 powder                                                                              7                                                       ______________________________________                                    

The zirconia curing composition having the composition described abovewas used to join a zirconia heating element having a shape shown in FIG.8(a) and (b) and a molybdenum disilicide lead member.

However, some cracks were observed in the cured portion of the zirconiacuring composition after drying. Furthermore, when the composition wascalcined at a temperature of 1,400° C., cracks developed and one side ofthe heating structure exhibited separation of the heating element bodyfrom molybdenum disilicide. Thus, the resulting product could not beused as a heating structure.

COMPARATIVE EXAMPLE 8

In order to join a heating element having a shape of Example 12 and aplatinum wire, a zirconia curing composition was used wherein 5 parts byweight of polyvinyl alcohol were further added to the zirconia curingcomposition used in Comparative Example 7 in order to reinforce the tackstrength and dry strength. Thus, a heating structure was formed.

This heating structure was calcined and thereafter used as a heatingelement. When retention for one hour at 2,000° C. was repeated threetimes, it seems that contact resistance generated due to cracks in thecured product of the zirconia curing composition. The lead membercomposed of the platinum wire therefore generates abnormally high heatand the platinum wire fused as a consequence.

What is claimed is:
 1. A process for producing a laminate-shapedzirconia refractory heating element which generates heat by passing acurrent or by induction heating, said process comprising the steps ofincorporating a flexible binder in a filler composed of zirconia fibersand a stabilizer containing zirconia powders; forming the mixture into arefractory sheet having flexibility at room temperature; windinglylaminating said refractory sheet in the form of a roll or folding it inthe form of a multilayer; and firing the laminate.
 2. The processaccording to claim 1 wherein in laminating said refractory sheet, theattaching side of a lead wire for passing a current is fixed to therefractory sheet, and thereafter said sheet is laminated.
 3. The processaccording to claim 1 or 2 wherein an emulsion of a vinyl acetate resinor polyacrylate and at least one plasticizer selected from ethyleneglycol, glycerin, and dibutyl phthalate are used as said flexiblebinder.
 4. The process according to claim 1 wherein there is used from20 to 50 parts by weight of the binder based on 100 parts by weight ofthe filler composed of from 5 to 80 parts by weight of the zirconiafibers and from 20 to 95 parts by weight of the stabilizer containingzirconia powders.